A reciprocating engine to be employed in a motor vehicle, a motorcycle, an agricultural machine, a marine vessel or the like requires a crankshaft to extract power by converting reciprocating motions of pistons to rotational motion. There are two types of crankshafts: the type manufactured by die forging and the type manufactured by casting. Especially when high strength and high stiffness are required, forged crankshafts manufactured by die forging are often employed.
A forged crankshaft is generally produced by using a billet as a starting material. The billet is circular or square in cross section, and the cross-sectional area is constant throughout the length. A method for producing a forged crankshaft includes a preforming step, a die forging step, and a trimming step that are to be executed in this order. After the trimming step, a coining step may be executed if needed. Typically, the preforming step includes a rolling step and a bending step, and the die forging step includes a rough forging step and a finish forging step.
FIGS. 1A to 1F are schematic diagrams showing a conventional method for producing a common forged crankshaft. A crankshaft 1 shown in FIG. 1F is a four-cylinder eight-counterweight crankshaft to be mounted in a four-cylinder engine. The crankshaft 1 includes five journals J1 to J5, four pins P1 to P4, a front part Fr, a flange Fl, and eight crank arms (hereinafter referred to simply as “arms”) A1 to A8. The eight arms A1 to A8 connect the journals J1 to J5 respectively to the pins P1 to P4. All of the eight arms A1 to A8 have counterweights (hereinafter referred to simply as “weights”) W1 to W8, which are integrated with the arms A1 to A8, respectively.
In the following paragraphs, when the journals J1 to J5, the pins P1 to P4, the arms A1 to A8, and the weights W1 to W8 are each collectively referred to, a reference character “J” is used for the journals, a reference character “P” for the pins, a reference character “A” for the arms, and a reference character “W” for the weights.
In the production method shown in FIGS. 1A to 1F, the forged crankshaft 1 is produced as follows. First, a billet 2 with a specified length as shown in FIG. 1A is heated in a heating furnace (for example, an induction heating furnace, a gas atmosphere heating furnace or the like), and the heated billet undergoes a rolling step. In the rolling step, the billet 2 is rolled and reduced, for example, by grooved rolls. This is to distribute the volume of the billet 2 in the axial direction, and thereby, a rolled blank 3, which is an in-process material, is obtained (see FIG. 1B). Next, in a bending step, the rolled blank 3 is partly reduced from a direction perpendicular to the length direction. This is to distribute the volume of the rolled blank 3, and thereby, a bent blank 4, which is a next in-process material, is obtained (see FIG. 1C).
Next, in a rough forging step, the bent blank 4 is reduced by a pair of an upper die and a lower die, and thereby, a rough forged blank 5 is obtained (see FIG. 1D). The rough forged blank 5 is roughly in the shape of a crankshaft (finished product). In a finish forging step, the rough forged blank 5 is reduced by a pair of an upper die and a lower die, and thereby, a finish forged blank 6 is obtained (see FIG. 1E). The finish forged blank 6 has a shape in agreement with the shape of the finished crankshaft. During the rough forging and the finish forging, excess material flows out through a space between the mutually facing parting faces of the dies, which results in formation of flash. Accordingly, the rough forged blank 5 and the finish forged blank 6 have great flash B on the periphery.
In a trimming step, for example, while the finish forged blank 6 is nipped and held by a pair of dies, the finish forged blank 6 is punched by a cutting die. Thereby, the flash B is removed from the finish forged blank 6, and a forged blank with no flash is obtained. The forged blank with no flash has substantially the same shape as the forged crankshaft 1 shown in FIG. 1F.
In a coining step, main parts of the forged blank with no flash are slightly pressed by dies from above and below so that the forged blank with no flash can have the exact size and shape of the finished product. The main parts of the forged blank with no flash are, for example, shaft parts such as the journals J, the pins P, the front part Fr, the flange Fl and the like, and further, the arms A and the weights W. In this way, the forged crankshaft 1 is produced.
The production method shown in FIGS. 1A to 1F is applicable not only to production of a four-cylinder eight-counterweight crankshaft as shown in FIG. 1F but also to production of any other crankshaft. For example, the production method is applicable to a four-cylinder four-counterweight crankshaft.
In a four-cylinder four-counterweight crankshaft, only some of the eight arms A1 to A8 incorporate a weight W. For example, the front first arm A1, the rearmost eighth arm A8 and the central two arms (the fourth arm A4 and the fifth arm A5) incorporate a weight W. The other arms, namely, the second, the third, the sixth and the seventh arms A2, A3, A6 and A7 do not have a weight. Such arms without a weight will hereinafter referred to as “unweighted arms”. These unweighted arms are oval.
Other crankshafts, for example, crankshafts to be mounted in three-cylinder engines, in-line six-cylinder engines, V-type six-cylinder engines, eight-cylinder engines and others can be produced by the same production method. It is noted that, when adjustment of the placement angles of the pins is necessary, a twisting step is added after the trimming step.
In recent years, there has been a need for weight reduction of reciprocating engines, particularly those for motor vehicles, in order to improve the fuel economy. Accordingly, there is also an ever-increasing demand for weight reduction of crankshafts to be mounted in reciprocating engines. Techniques to reduce the weight of a crankshaft are disclosed in Japanese Patent Application Publication No. 2012-7726 (Patent Literature 1) and Japanese Patent Application Publication No. 2010-230027 (Patent Literature 2).
Patent Literatures 1 and 2 teach an arm having a hole made in the journal-facing surface and teach a method for producing a crankshaft with the arm. The hole of the arm is made to lie on a straight line connecting the axis of the journal and the axis of the pin (which will be hereinafter referred to as an “arm centerline”), and the hole extends large and deep toward the pin. This arm is reduced in weight by the weight corresponding to the volume of the hole. The weight reduction of the arm leads to a weight reduction of the weight paired with the arm, thereby resulting in a reduction in weight of the whole forged crankshaft. Regarding the arm having a hole, in a region near the pin, both side portions across the arm centerline are thick, which ensures stiffness (torsional stiffness and bending stiffness).
Forming a recessed portion in the journal-facing surface of the arm while keeping both side portions of the arm thick as described above leads to weight reduction and ensuring of stiffness.
It is, however, difficult to produce such a forged crankshaft with such uniquely shaped arms by a conventional production method. The reason is as follows. When a recess is to be formed in the surface of an arm in the die forging step, the draft of the die will become a reverse draft at the site of the recess, and therefore, the forged blank will not be able to be removed from the die.
To avoid such situations, in the production methods disclosed in Patent Literatures 1 and 2, the following process is carried out. In the die forging step, the arm is shaped to be small with no recess formed in the surface of the arm, and after the trimming step, a punch is pushed into the surface of the arm so that the imprint made by the punch forms a recess.
In the crankshaft shown in FIG. 1F, all of the arms A and the weights W integrated therewith have the same shape. Practically, however, the arms A and the weights W integrated therewith may be different from one another in shape as needed. Japanese Patent Application Publication No. 2007-71227 (Patent Literature 3) and Japanese Patent Application Publication No. 2014-40856 (Patent Literature 4) disclose techniques for this.
Patent Literature 3 discloses a four-cylinder eight-counterweight crankshaft including a flywheel disposed at an end. In the crankshaft, the arms incorporating a weight are different from one another in the thickness and the center of gravity of the arm and in the mass of the weight. Accordingly, it is possible to reduce the thicknesses of the arms that need to have only low stiffness while ensuring the minimum necessary stiffness to each of the arms, thereby resulting in a reduction in weight.
Patent Literature 4 discloses a crankshaft for a multicylinder engine, the crankshaft including a flywheel disposed at an end. In the crankshaft, an arm that is less distant from the flywheel has higher bending stiffness and higher torsional stiffness than an arm that is more distant from the flywheel. Also, it is preferred that the arms are different from one another in the bending stiffness and in the torsional stiffness. Accordingly, it is possible to attain a reduction in weight while suppressing flexural vibration and torsional vibration.
In a case where the arms incorporating a weight have different arm shapes and different weight shapes, what portion of the arm needs to have high stiffness differs from arm to arm, depending on the shape. Specifically, an arm may need to have high stiffness in the region near the pin, while another arm may need to have high stiffness in the region near the journal.